1. Field of the Invention
The invention relates to the field of the optical measurement of electrical quantities. It concerns a fiber-optic sensor, comprising
(a) a light source; PA1 (b) a piezoelectric sensor element; PA1 (c) a first double-mode fiber having an input end and another end, in which fiber the LP.sub.01 fundamental mode and the even LP.sub.11 mode can propagate, and which fiber is fixed at least partially to the sensor element in such a manner that a dimensional alteration of the sensor element in an electric field leads to a length alteration in the fiber; and PA1 (d) means for measuring the field-dependent length alteration of the fiber; PA1 (e) the light source is a multimode laser diode; PA1 (f) the measuring means comprise a second double-mode fiber; PA1 (g) the parameters of the two double-mode fibers and thus the relative path differences (Delta L.sub.1, Delta L.sub.2), which the modes accumulate in the individual double-mode fibers, are tuned to the coherence properties of the light source so that the interference contrast (V) for the said path differences (Delta L.sub.1, Delta L.sub.2) and for the sum of these path differences (Delta L.sub.1 +Delta L.sub.2) is in each instance approximately equal to zero and for the difference of the said path differences (Delta L.sub.1 -Delta L.sub.2) adopts an absolute or relative maximum; PA1 (h) the other end of the first double-mode fiber is mirrored; PA1 (i) for the transmission of light between the light source and the measuring means on the one hand and the input end of the first double-mode fiber on the other hand, a monomode fiber is provided; and PA1 (j) a fiber coupler or beam splitter is provided for the coupling-in of light from the light source into the monomode fiber and for the coupling-out of light reflected at the other end of the first double-mode fiber out of the monomode fiber into the second double-mode fiber. PA1 only a single monomode fiber is required for the transmission of the light between the light source and the measuring means on the one hand and the first double-mode fiber or sensor fiber; PA1 as the light must pass through the sensor fiber twice, the phase modulation caused by the piezoelectric sensor element and thus also the sensitivity of measurement of the arrangement are doubled.
Such a fiber-optic sensor is known, for example, from EP-A1-0,433,824.
2. Discussion of Background
Fiber-optic sensors for the measurement of electric fields and voltages have already been described in various publications such as, for example, the European Patent Applications EP-A1-0,316,619 and EP-A1-0,316,635 or the articles by K. Bohnert and J. Nehring in Appl. Opt. 27, pp. 4814-4818 (1988), or Opt. Lett. 14, pp. 290-292 (1989).
The measurement principle employed in this case is based on the inverse piezoelectric effect in materials with selected crystal symmetry. The temporally periodic dimensional alteration which is experienced by an appropriate piezoelectric body in an alternating electric field is transmitted to a glass fiber fixed to the body. The length alteration of the fiber is then proportional to the field or voltage amplitude and is measured by interferometry and evaluated.
It is possible to use various types of glass fiber interferometers for the interferometric measurement. On account of its simplicity, of these types the double-mode fiber interferometer known from the article by B. Y. Kim et al., Opt. Lett. 12, pp. 729-731 (1987) is of particular interest. In this interferometer, the parameters of the sensor fiber are selected so that precisely two modes (the LP.sub.01 fundamental mode and the even LP.sub.11 mode) can propagate in the fiber.
In the double-mode fiber interferometer, light from a coherent light source, e.g. a laser diode, is passed through a double-mode fiber, which is fixed to a piezoelectric sensor element for the electric field E. The two modes are excited by the light and propagate differently in the fiber. At the fiber end, it is then possible to observe an interference pattern, which arises from the superposition of these two modes. In this case, a length alteration of the fiber leads to a differential phase shift between the two modes, which is expressed in a corresponding alteration of the interference pattern.
The interference pattern exhibits two mutually adjacent substructures, which are detected by two detectors (e.g. in the form of photodiodes). At their output, there are two signals V.sub.11 and V.sub.12 which are phase-shifted by 180.degree.: EQU V.sub.11 =(1/2)V.sub.0 (1+a*cos .phi.(t)) (1) EQU V.sub.12 =(1/2)V.sub.0 (1-a*cos .phi.(t)) (2)
where .phi.(t)=A*sin .omega.t.theta.(t). The phase shift .phi.(t) between the two modes is thus composed of a temporally periodic component A*sin .omega.t caused by the alternating field to be measured (in this case, A is proportional to the amplitude of the field) and an arbitrary phase term .theta.(t), which can likewise alter with time, for example in consequence of temperature-dependent fluctuations of the fiber length. Finally, V.sub.0 is proportional to the optical power and a is a measure of the interference contrast.
The sought term A*sin .omega.t is frequently obtained using a homodyne detection method with active phase compensation from the output signals of the detectors (for a fiber-optic sensor with a-single-mode fiber, see in this connection: D. A. Jackson et al., Appl. Opt. 19, pp. 2926-2929 (1980); a corresponding fiber-optic sensor with a double-mode fiber is described in the initially cited European Application EP-A-0,433,824). In this method, the sensor fiber is additionally guided via a piezoelectric modulator. By means of this modulator 4, the phase shift .phi.(t) is regulated to +(pi/2) or -(pi/2) (modulo 2pi). To this end, the modulator is a component part of a control circuit which comprises the detectors, a subtractor and a quadrature regulator and which regulates the difference voltage EQU V=V11-V12=V0*a*cos .phi.(t) (3)
to zero in each instance.
The two components A*sin .omega.t and .theta.(t) of the phase shift are both balanced by the modulator via a corresponding (opposite) length alteration of the fiber in- a direct manner. The voltage present at the modulator includes a slowly varying component, which is proportional to .theta.(t), and a periodic component, which is proportional to A*sin .omega.t. The sought component A*sin .omega.t is filtered out via a high-pass filter and can be picked off at the signal output. The output signal is, as a result of this, independent of any possible fluctuations of the laser intensity (i.e. V.sub.0) and of the interference contrast a.
On the other hand, the sought term A*sin .omega.t can also be obtained by means of a homodyne method in which a carrier phase modulation is generated, or by means of a synthetic heterodyne method (in this connection, see: A. Dandridge et al., IEEE J. of Quantum Electronics QE-18, 1647 (1982); J. H. Cole et al., IEEE J. of Quantum Electronics QE-18, 694 (1982); E. L. Green et al., IEEE J. of Quantum Electronics QE-18, 1639 (1982)). In these methods, a modulator is likewise provided. The latter is driven by an oscillator; in this case, the oscillator signal is also passed to a demodulating electronic system. In both methods, it is sufficient to evaluate only one of the two aforementioned interference patterns. They differ only by the nature of the required demodulating electronic system.
In a series of practical applications of the sensor (e.g. in voltage measurement in outdoor installations), relatively large spacings may occur between the actual sensor head and the sensor electronic system (10 m to a few 100 m). It is inexpedient to bridge these spacings with the double-mode fiber itself, since the influence of external disturbances (temperature fluctuations, mechanical vibrations, etc.) is correspondingly enlarged with increasing fiber length and the signal/noise ratio is impaired. The light supply from the laser diode to the interferometer and the return of the output signals of the interferometer should rather take place via separate glass fibers, which are not a component part of the interferometer.
In the above-described homodyne method using an active phase modulator, it would, however, in addition to the connecting glass fibers, also be necessary as well to provide an electrical connection between the sensor electronic system and the sensor head to drive the modulator. The attractiveness of a sensor operating with this type of interferometer would thus be very limited.
Accordingly, it was proposed in two older German Patent Applications (file references P 41 14 253.5 and P 41 15 370.7) to provide, in place of the known active signal detection, which requires an additional modulator in the measurement fiber with a corresponding electrical supply, a passive signal detection which is based on the Guoy effect (in this connection, see: S. Y. Huang et al., Springer Proc. in Physics, Vol. 44 "Optical Fiber Sensors", pp. 38-43, Springer Verlag Berlin, Heidelberg (1989)), i.e. the phase difference between the interference patterns of the close and remote field: the substructures of the close and remote field (4 in total) are in this case separated in the sensor head by optical means, and can be transmitted via separate glass fibers to a remote electronic evaluation system. There, using at least three of these four substructures, the desired information can be obtained via the length alteration of the measurement fiber. Using this proposed solution, a complete electrical separation is indeed achieved between the sensor head and evaluating electronic system. However, this advantage is acquired at the expense of a relatively complex optical system and electronic system. Over and above this, it is necessary to use a monomode laser diode, which demands particular measures for the suppression of the light back-scatter from the sensor into the diode.